(1) Field of the Invention
The present invention relates to a solid-state imaging device, and in particular to a solid-state imaging device having an improved read-out transistor portion which reads out signal charge from a photodiode, and to a manufacturing method thereof.
(2) Description of the Related Art
Conventionally, solid-state imaging devices such as a Charge-Coupled Device (CCD) solid-state imaging device and a metal-oxide semiconductor (MOS) solid-state imaging device are used for various image input apparatuses such as video cameras, digital still cameras and facsimiles. The solid-state imaging devices are mainly used for portable apparatuses and low-voltage drive is required for the solid-state imaging devices. A solid-state imaging device responding to this requirement is disclosed, for example, in Japanese Laid-Open Patent Application No. 2000-150847.
FIG. 1 is a circuit diagram showing a structure of a conventional MOS-type solid-state imaging device.
The solid-state imaging device includes an image area 201 in which n×m unit cells 200 are arranged in a two-dimensional array, a first vertical signal line 202 for transmitting signal voltages of the unit cells 200 on a column-by-column basis to a signal processor 205, a vertical shift register 203 which selects the unit cells 200 on a row-by-row basis, a load transistor group 204, and the signal processor 205 which holds the signal voltages transmitted through the first vertical signal line 202 and cuts noise, a horizontal shift register 206 which selects the unit cells 200 on a column-by-column basis, a horizontal signal line 207 for transmitting the signal voltage outputted from the signal processor 205 to an output amplifier 208, and the output amplifier 208.
Each of the unit cells 200 is made up of a photodiode 211 which converts an optical signal into signal charge, a read-out transistor 212 which reads out the signal from the photodiode 211, an amplifying transistor 213 which amplifies signal voltage of the photodiode 211, a reset transistor 214 which resets the signal voltage of the photodiode 211, a vertical selecting transistor 215 which selects a row from which the amplified signal voltage is to be read out, and a floating diffusion (FD) unit 216 which detects the signal voltage of the photodiode 211.
FIG. 2 is a cross-sectional diagram showing a structure of the unit cell 200 (a cross-section in the vicinity of the read-out transistor 212).
The unit cell 200 includes an n-type signal accumulation region 222, a p-type surface shielding region 223, an n-type drain region 224 and a p-type punch-through stopper region 226 that are formed in a p-type semiconductor substrate 221. A gate electrode 225 of the read-out transistor 212 is also formed on the semiconductor substrate 221.
The signal accumulation region 222 operates as the photodiode 211 and accumulates the signal charge that is obtained through a photo-electrical conversion.
The surface shielding region 223 is positioned in a surface of the semiconductor substrate 221 and next to the gate electrode 225, and prevents the accumulation of noise charge in the signal accumulation region 222 by shielding the noise charge generated in the surface of the semiconductor substrate 221.
The drain region 224 is positioned opposite the surface shielding region 223 across the gate electrode 225 in the surface of the semiconductor substrate 221, and operates as the FD unit 216.
The punch-through stopper region 226 is positioned closer to a rear surface of the semiconductor substrate 221 than the drain region 224, and prevents the punch-through, in other words, prevents the signal charge accumulated in the signal accumulation region 222 from being read out to the drain region 224 without a control by the gate electrode 225.
Here, the surface shielding region 223 and the punch-through stopper region 226 have an impurity concentration that is one-digit greater than that of the semiconductor substrate 221.
In a solid-state imaging device having the configuration described above, punch-through is prevented by the punch-through stopper region 226 even when the impurity concentration in the p-type region below the gate electrode 225 is decreased as much as 1×1016 to 1×1015 cm−3. Therefore, channel modulation by the gate voltage becomes more effective by decreasing the impurity concentration of the p-type region below the gate electrode 225, and the gate voltage can be lowered. In other words, a solid-state imaging device driven under low voltage can be realized.